Snap-action leaf spring switch



Nov. l, 1966 T. Y. KvoRsGREN 3,283,109

SNAP-ACTION LEAF SPRING SWITCH Filed Nov. s, 1964 2 sheets-sheet 1 j w l FWG. 2

Nov. 1, 1966 l T. Y. KoRsGREN 3,283,109

SNAPACTION LEAF SPRING SWITCH Filed NOV. 30, 1964 2 Sheets-Sheet 2 United States Patent O 3,283,109 SNAP-ACTION LEAF SPRING SWITCH Theodore Y. Korsgren, Cary, Ill., assignor to International Telephone and Telegraph Corporation, New York, N Y., a corporation of Maryland Filed Nov. 30, 1964, Ser. No. 414,735 7 Claims. (Cl. 200-166) This invention relates to electrically conductive leaf springs and more particularly to bridges for reducing or controlling the ydifferential movement of such springs.

Many electrical devices use leaf `springs for carrying electrical current. An actuated point on this type of leaf spring moves over a discrete distance (sometimes called the differential movement) and causes the contacts to open or close an electrical circuit responsive to an excursion of a moving part ony an associated control device, sometimes called an actuatorf The elasticity of the leaf spring causes it to return to normal when the moving part returns to normal. This, in turn, returns the electrical circuit to normal. The invention does not require any particular type of leaf spring; however, a monostable snap acting spring will be described, to provide a concrete illustration.

Generally, the actuator for moving the spring has Va somewhat critical design characteristic. F or example, the actuator could be a relay which must operate only when an applied voltage or current exceeds a given value for a specific length of time. Or, the actuator could be a sensor controlled device which must operate only when the ambient temperature, for example, reaches a certain degree. Those skilled in the art will readily perceive other uses for a leaf spring contact and other reasons why the actuator may tend to have critical characteristics.

The one thing which most of these actuators have in common is a need for extremely good reliability. A relay, snap action switch, or other control device does not control if it fails to respond reliably at a predetermined threshold level. It-s failure to control can, in turn, result in a costly failure or destruction of extremely valuable equipment. For any given reliability requirement, the spring must have a given movement differential, resilience, and other characteristics. Thus, the actuator will rfunction properly only if these spring characteristics are both suiiiciently uniform and non-critical.

In the past, these leaf springs have sometimes tended to require too much mechanical movement. A desirable leaf spring would have a short travel and yet maintain great resilience. This way, the electrical contacts on the leaf spring may :be operated responsive to the light touch and small motion of an extremely sensitive actuator. However, when efforts are made to decrease the length of actuator travel, the resilience of the `spring blade increases and becomes the critical and limiting factor. Hence, there is generally a trade-off wherein some excessive spring travel is tolerated in order to hold the spring resilience within reasonable limits. Or, some increase in resilience is tolerated in order to hold spring travel within reasonable limits. Either way, the leaf spring is less than ideal, requires actuator design compromises, and tends to reduce actuator reliability. Conversely, to carry an increased current, it is sometimes desirable to increase the gap across open contacts without changing the movement differential required to close the contacts.

Accordingly, an object of this invention is to provide new and improved leaf springs for use as part of current carrying electrical contacts. In this connection, an object is to reduce the motion required to cause a contact travel Without increasing the leaf spring resilience. Hence, an object is to provide a leaf spring construction where the travel and resilience may be varied independently of each other. More specifically, an object is to either reduce the "ice differential without affecting the open contact gap or to increase the gap without affecting the differential.

Another .object of the invention is to increase the sensitivity and reliability of electrical devices using leaf springs. In particular, an object is to provide new and improved snap action switches for use with automatic sensors.

Yet another object lof the invention is to accomplish these and other objects without materially increasing the costs of either the leaf springs or the associated actuators. Quite the contrary, an object is to reduce the cost of both the leaf springs and the associated actuators. In this connection, an object is to reduce such costs .by making the leaf spring members on conventional machine tools.

These and other objects are accomplished by means of a bridge on a multi-tine, leaf spring assembly. In the exemplary construction :described herein, the assembly is supported at one end of a tine held in tension and biased to a monostable condition by at least one other tine held in compression. When a force is applied to the tension tine, the spring snaps to an oif-normal position from which it returnsowing to its self-bias-after the force is removed. According to the invention, a bridge member loosely rides upon and at least partly surrounds the tine held in tension to preclude the application of a force on a single point to cause a flexing of the tension member. This way, the tension tine is `flexed by :forces which can only Ioccur near the ends of the bridge member. This has the effect of restricting the actuator travel without changing the contact travel.

The above mentioned and other objects and features of this invention and the manner of obtaining them will become more apparent, and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a bottom plan view which shows an exemplary usage of the leaf spring as part of a snap action switch;

FIG. 2 is a cross sectional view of the switch taken along the line 2 2 of FIG. 1;

FIG. 3 is a top plan view of the leaf spring assembly, per se;

FIG. 4 is a side elevation view of the leaf spring taken along line 4-4 of FIG. 3;

FIG. 4A shows how the tension-tine bends without the bridge member;

FIG. 4B shows how the tension-tine bends with the bridge member;

FIG. 5 is a cross sectional view of a iirst embodiment of the invention taken along line 55 of FIG. 3;

FIG. 6 is a cross sectional view of a second embodiment of the invention also taken along the line 5 5 of FIG. 3;

FIG. 7 is a top plan view showing of a continuous strip of leaf spring bridge elements produced by an automatic punch press;

FIG. 8 is a top plan view of a single bridge elemen after it has been cut from the strip of FIG. 7;

FIG. 9 is a cross sectional view of the bridge element taken along line 9 9 of FIG. 8; and

FIG. l0 is a side elevation view of the bridge element taken along line 10-10 of FIG. 8.

The snap action switch of FIGS. 1 and 2 includes a plastic base 20 having a leaf spring assembly 21 mounted there-on. One end of the spring assembly 21 is supported on the base 20 at lpoint 22. The other end 23 of the spring assembly 21 is free to move between the upper and lower positions. In the upper or normal position, the end 23 makes electrical contact with a stationary contact 24. In the lower or off-normal position, the end 23 makes electrical contact with another stationary contact 25. Electrical wires (not shown) are attached to these contacts via terminal lugs such as 26.

The leaf spring assembly 21 is lhere shown as a flat.

blade -of spring material having three tines 27, 28, 29 (FIG. 3). This spring can be made from a cleaned and hardened piece of beryllium copper. The center tine 28 is supported at the end 22 to provide for a mechanical motion in a fixed-free mode. The two outer tines 27, y29 bear against a fulcrum plate 30 (FIG. l) at points 31, 32, respectively. This plate could be made of brass which is thick enough to preclude any significant'motion at the fulcrum points 31, 32.

The proportions of ythe parts are such thatthe center tine 28 is held in tension and the outer two tines 27, 29 are held in compression. This compression-tension arrangement provides for a snap action in a well known manner.

The snap action switch of FIGS. 1, 2 includes an actuator (not shown) arranged to exert a downward (as viewed in FIG. 2) force upon `the tension leaf spring tine 28. This force is represented by the arrow A1. When this force bears down on the tension tine member 28, the contact end 23 moves over center, and the leaf spring assembly 21 snaps to its off-normal position which closes contacts 23, 25 and opens contacts 23, 24. The spring is inherently monostable so that it returns to normal when the force A1 is removed. In the normal position contacts 23, 24 'close and contacts 23, 25 open.

In keeping with the invention, bridge means 40 is added to the tension tine member 28 between the point 22 of support and the contact end of the spring. The force A1 is applied t the bridge means. The location and operation of the bridge is shown best in FIGS. 3-6. The design of the bridge itself may be understood best from a study of FIGS. 7-10. The purpose of the bridge is to span any sag in tine 28 (FIG. 4B) and thereby tend to reduce the differential.

In greater detail, `the bridge members may be made on a punch press as a continuous strip of material which looks somewhat as shown in FIG. 7. Each section of this strip, such as 40, constitutes a single bridge element. After the punch press has completed the strip, it is sheared, sawed or otherwise cut at the end of each bridge section. For example, the strip of FIG. 7 may be cut at 41, 42 to make the single bridge element which is shown in FIGS. 8-10.

Conveniently, one end of the bridge is shaped to form a tab section 43 and two depending lugs 44, 45. The tab 43 is bent downwardly and fitted into a hole 46 formed near the supported end 22 of the tension tine member 28. However, this particular arrangement is exemplaryV only. Neither the tab, lugs, nor hole are required as long as the general purpose of anchoring the bridge is accomplished. The orientation and dimensions of bridge element 40 is such that it fits loosely over the spring tine 28 and extends from near the support 28 toward the contact end.

The bridge lugs 44, 45 t over either side of the spring 28 to hold the bridge against both sidewise and rotational motion. Then, the lugs are rolled (as shown at 47,48) to secure the bridge 40 to the spring. As shown at 50 (FIG. the ends of theV roll may come close to but avoid actual contact with the spring 28. This provides a freedom,'within restricted limits, of flexing motion between the bridge and spring. Or, as shown at 51 (FIG. 6), the lugs may be rolled far enough to come into actual contact with the spring 28 to hold the bridge rmly against the spring member. However, in the embodiment of FIG. 6, care must be taken `to avoid distortion of the spring 28. This is important because internal stresses would otherwise form in the spring and cause a premature failure owing to metal fatigue.

Stated otherwise, a primary advantage of the invention is the freedom which it provides from internal stresses within the spring 28 which could result in an early metal 4 fatigue. These internal stresses could result fromstress starters forming at the ends of any areas in the spring which might be stitfened by embossments or rails `on or attached to the spring. The embossment or rails is what must be avoided in the embodiment of FIG. 6.

The function of the bridge is to reduce thek differential or mechanical motion required to snap the1switch. This function may be understood best from a study of FIGS. 4A and 4B. In these figures, dashed lines are used to show the normal position of the tensioned tine 28. A somewhat arcuate shaped solid line is used to show symbolically the olf-normal position of tine 28. The arcuate shape, shown here, is idealized to illustrate the operation;

it does not necessarily represent any actual shape of the tlexed spring. The supported end of the tensioned tine is indicated by short, slanting ground lines.

If no bridge member is present the spring exes about a point P where the bending force A1 is applied. This means that the structure applying the force must move over the distance D1,\FIG. 4A. This is the movement distance called the diiferential.

If the bridge 40 is present (FIG. 4B), the spring (solid line) ideally exes i-n exactly the same way that it exes when the bridge is absent (FIG. 4A). However, the bending force is transmitted through the bridge to two points on the spring. The. Ibridge extends over lthe point where the spring sags the most. Thisreduces the movement of the structure applying the force to the distance D2. Or stated another way,` t-he brid-gereduces the differential from the distance D1 to the distance D2.

An alternative to the embodiment shown herein is to support the bridge on the end of the actuator structure for applying the for-ce A1. In this alternative embodiment, .the lbridge should be loosely supported (on the end of a pin plunger, for example) so that it may adopt any attitude which Iallows the spring to follow its normal mode of bending.

An yadvantage of the invention is that the spring may carry greater -current for any given differential. If the current remains the same before and after the bridge is used, the differential is reduced. If -the differential remains the same, the current carrying capacity is increased because the contact gap is increased.

While the principles of the invention Vhave `been described above in connection with specific apparatus and applications, it is to be understood that this description is made only by Way of example and not as -a limitation on the scope of ,the invention.

I claim:

1. A multi-tine, leaf spring assembly having at least one tine member held in tension and one other tine member held in compression, said assembly being supported at one end of said tension tine member, said compression tine member biasing said assembly for monostable snap action responsive to an application of force upon said tension member, and generally inflexible bridge member mounted on said tension member between the ends thereof and extending along a substantial lengt-h of said tension member for receiving the applied force for reducing the differential required to move said tension member without affecting the resilience of said spring.

2. The assembly of claim 1 wherein said lbridge member is made with two depending lugs which fit over either side of -the tension member and are formed to secure the bridge to the tension member without distorting said' spring.

3. The assembly of claim 1 wherein the formed ends of said lugs do not touch said spring.

4. The assembly of claim 1 wherein the formed ends of said lugs come into contact with said spring.

5. An electrical current carrying leaf spring assembly having at least one blade member mounted adjacent one end and contact means Iattached to the other end, means for applying an actuator force upon said spring blade intermediate its two ends to Hex said blade and cause it rto move from a normal to an off-normal position, and a member has a tab depending through said hole to restrict generally inexible bridge member loosely mounted on movement of said bridge along said tension member. said tension member between the ends thereof and oriented to extend along the length of said blade member for References Cited by the Examiner applying said actuator force at nwo points on said blade 5 UNITED STATES PATENTS to reduce the differential required io move said member. 3,056,866 10/1952 Kal-leen 200 67 6. The assembly of claim 5 and means for precluding a distortion of said spring by said bridge member. ROBERT K. SCHAEFER, Primary Examiner.

7. The assembly of claim 2 wherein said tension mem- KATHLEEN H CLAFFY Examiner.

f 'th y11 1 d d 'd 10 ber 1s ormed w1 a o e near one en and sai bri ge H. O. JONES Assistant Examiner. 

1. A MULTI-TINE, LEAF SPRING ASSEMBLY HAVING AT LEAST ONE TINE MEMBER HELD IN TENSION AND ONE OTHER TINE MEMBER HELD IN COMPRESSION, SAID ASSEMBLY BEING SUPPORTED AT ONE END OF SAID TENSION TINE MEMBER, SAID COMPRESSION TINE MEMBER BIASING SAID ASSEMBLY FOR MONOSTABLE SNAP ACTION RESPONSIVE TO AN APPLICATION OF FORCE UPON SAID TENSION MEMBER, AND GENERALLY INFLEXIBLE BRIDGE MEMBER MOUNTED ON SAID TENSION MEMBER BETWEEN THE ENDS THEREOF AND EXTENDING ALONG A SUBSTANTIAL LENGTH OF SAID TENSION MEMBER FOR RECEIVING THE APPLIED FORCE FOR REDUCING THE DIFFERENTIAL REQUIRED TO MOVE SAID TENSION MEMBER WITHOUT AFFECTING THE RESILIENCE OF SAID SPRING. 